The invention is directed to high thermal expansion glass compositions for use in castable tape compositions to fabricate multilayer circuits for the ball grid array (BGA) application.
Recent packaging technologies concerning ceramic materials have demanded a higher packing density, better performance, and a lower cost. The low temperature co-fired ceramic tape (LTCC) technology has been regarded as a key solution, which can meet all of these requirements. LTCC is a well-known technology for combining high conducting metallization (silver and gold) with reliable ceramic dielectric layers for dense packaging of IC circuits. Generally, LTCC substrate consists of glass and ceramic. Glass allows a low temperature firing below 900xc2x0 C., while ceramic (as a filler) gives high mechanical strength and dimensional stability as a result of certain interactions with glass. In most cases, the design of glass is more important than the choice of ceramic materials particularly when specific functions are required. The selection of glass determines compatibility of the resultant substrates with other contacting materials such as conductors and passive devices.
The LTCC materials have been carefully designed also for the BGA applications, where PWB materials having a high TCE (Temperature Coefficient of Expansion) of 12-20 ppmxc2x0 C.xe2x88x921 are integrated with LTCC materials via solder joints. As a potential solution to achieve a better integrity between the dissimilar materials, a new ceramic substrate having a high TCE above 9 ppmxc2x0 C.xe2x88x921 has been demanded. The high TCE LTCC can reduce thermal stress at the solder joints, leading to better thermal cycle behavior without substantial cracking or electrical failures.
The present invention fills such a need by providing new glass compositions which exhibit high thermal expansion. The resultant glass-ceramic body after firing exhibits good strength and silver compatibility. In addition, the new glasses allow for low temperature firing, i.e., below the melting point of silver which broadens the processing latitude of present day systems.
The present invention relates to an alkali-containing magnesium borosilicate glass composition comprising, in mole %, 10-25% SiO2, 10-25% B2O3, 5-10% BaO, 40-65% MgO, 0.5-3% ZrO2, 0.3-3% P2O5, and 0.2-5% M2O where M is selected from the group of alkali elements and mixtures thereof. The preferred alkali elements are Li, Na and K.
The invention is further directed to a castable dielectric composition comprising a dispersion of finely divided solids comprising, based on solids: (a) 50-90 wt. % the glass composition as described above; (b) 10-50 wt. % ceramic filler; both dispersed in a solution of (c) an organic polymeric binder; and (d) a volatile organic solvent.
The invention is still further directed to the castable dielectric composition described above used in a method of forming a high TCE LTCC green tape by casting a thin layer of the castable dispersion onto a flexible substrate and heating the cast layer to remove the volatile organic solvent.